Lower aldehydes, typically formaldehyde, acetaldehyde, etc., are harmful gases having a characteristic irritating odor. For formaldehyde, an acceptable air concentration level of 0.5 ppm has been tentatively provided, and acetaldehyde has been designated as a malodorous substance in Japan.
Sources of formaldehyde include, among others, formaldehyde-producing plants, plants producing resins starting with urea, melamine, phenol or the like and formaldehyde, processing plants in which such resins are used and, further, factories manufacturing various products, for example building materials and furniture, using said resins. It is said that formaldehyde is generated from formalin used in hospitals as a disinfectant or by incomplete combustion in kerosene stoves and that collateral streams of tobacco or cigarette smoke also contains formaldehyde in fair amounts.
Acetaldehyde sources include plants for the production of acetaldehyde and derivatives thereof and, further, acetaldehyde is formed upon heat treatment of sewage sludge and is contained also in the mainstream of tobacco or cigarette smoke.
In recent years, the hazard and odor features of these lower aldehydes have become problems from the viewpoints of improved working environment and better living environment, and efficient hazard elimination measures have been eagerly demanded.
The adsorbents so far used for lower aldehydes include, among others, active carbon, activated clay, silica gel, activated alumina, and clay minerals and, among them, active carbon has been more often than not employed. These adsorbents, typically active carbon, as such, have drawbacks from the characteristics viewpoint; for example, they are poor in adsorbency for lower aldehydes, such as formaldehyde and acetaldehyde, and have a short service life.
As a measure for achieving improvements in these respects, it has already been proposed to have supported on such adsorbents a compound reactive with lower aldehydes. Examples of such compound are organic compounds such as hydrazines, aliphatic amines (Japanese Published Examined Patent Application No. 63-24413), aromatic amines (Japanese Published Unexamined Patent Application No. 60-132645) and ureas, and inorganic compounds such as ammonium salts, sulfites, oxides and hydroxides of alkali metals, alkaline earth metals, etc., iodine, bromine, and compounds of iodine or bromine with alkali metals or alkaline earth metals (Japanese Published Examined Patent Application No. 61-20330).
It has also been proposed that the above-mentioned adsorbents should carry a platinum group metal compound.
However, the adsorbents carrying an organic compound have problems from the following viewpoints: time-course stability, hazardousness, heat resistance and odor of the organic compound, among others. In particular, the adsorbent carrying aniline (Japanese Published Examined Patent Application No. 60-54095) is disadvantageous in that aniline itself has poor time-course stability in adsorbing lower aldehydes. Furthermore, aniline is a suspected carcinogen. For these reasons, it has been difficult to put such adsorbent into practical use.
The adsorbents carrying an inorganic compound are insufficient in the rate of adsorption of lower aldehydes occurring in low concentrations. As regards the catalyst carrying adsorbents, the catalyst is expensive and the elimination effect at ordinary temperature is low.
Thus, none of the prior art adsorbents can be regarded as being a satisfactory medium for elimination of lower aldehydes.
The object of the invention is to provide an adsorbent capable of eliminating lower aldehydes efficiently for a long period and which has an excellent heat resistance.